The present invention relates to a novel dilithium polymerization initiator solution useful for the production of a block copolymer having narrow molecular weight distribution and excellent symmetric property or a telechelic polymer having, equally at both terminals thereof, a functional group, and relates to a production process thereof.
An attempt has long been made to polymerize a block copolymer having an SBS structure or SIS structure by using a dilithium compound as a polymerization initiator. This attempt has not always lead to practical industrial production. One of the reasons resides in that a polymerization initiator cannot be constituted of a dilithium compound alone. A polymer obtained using a polymerization initiator solution composed of many components involves such a drawback as inferior mechanical properties for example tensile strength, because such a polymer inevitably has a widened molecular weight distribution, and because polymerization proceeds only at one of the polymerization starting points of the dilithium compound, thereby making the resulting polymer to inevitably contain a copolymer having a diblock structure. Another reason resides in that in order to heighten a solubility of the dilithium compound in a nonpolar solvent and moreover to prepare a polymerization initiator as another-component free as possible, use of a sufficient amount of a polar substance containing a nitrogen or oxygen atom is indispensable during the preparation step of the polymerization initiator. This imposed a limitation on the bonding structure of a polymer in the subsequent polymerization step of a conjugated diene, resulting in the production of only a polymer having a large 1,2-vinyl-bound content. In addition, the polymer solution thus obtained cannot be provided for the subsequent reaction without any convenience. For example, upon hydrogenation reaction of the polymer, a tertiary amine compound, which is a nitrogen-containing compound among such polar substances, causes damage to a catalyst for hydrogenation of a conjugated diene polymer so that the polymer solution cannot be provided as is for hydrogenation.
For example, JP-B-63-5403 (The term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d) discloses a process for producing a polymer having narrow molecular weight distribution by using a dilithium polymerization initiator. JP-B-1-53681 or JP-B-6-67990 discloses a dilithium polymerization initiator for obtaining a polymer having narrow molecular weight distribution. The techniques disclosed in them are for obtaining a dilithium polymerization initiator by reacting a monolithium compound with 1,3-diisopropenylbenzene in the presence of a tertiary amine.
JP-A-8-48709 (EP690075A1) (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined Japanese patent applicationxe2x80x9d), discloses an improvement of the above-described polymerization process using a dilithium polymerization initiator. As the polymerization initiator, described is an xcex1,xcfx89-dilithiopoly(conjugated diene) which is an oligomer obtained by reacting a monolithium compound with 1,3-diisopropenylbenzene in the presence of a tertiary amine and then reacting with a small amount of a conjugated diene. The object and means of the invention described above are to provide a polymer having narrow molecular weight distribution by forming an xcex1,xcfx89-dilithiopoly(conjugated diene) from the dilithium compound thus obtained and a small amount of the conjugated diene and carrying out polymerization using this conjugated diene oligomer as a polymerization initiator. This process needs complex steps. A number of patent official gazettes and references were introduced in it as a prior art.
In the above-described prior art, a tertiary amine is used as an essential component so that, as described above, the polymer solution cannot be provided for hydrogenation reaction as is. In addition, many restrictions are imposed on it, for example, use of an oligomer polymerization initiator to control the initiation of the polymerization reaction, a decrease in the amount of the conjugated diene at the initial stage of the reaction and use of the polymerization initiator at a predetermined concentration.
xe2x80x9cMacromolecules, 27, 2219-2255 (1994)xe2x80x9d discloses a lithium polymerization initiator synthesized using sec-BuLi which initiator is soluble in toluene, cyclohexane or n-hexane and that a block copolymer can be prepared using this polymerization initiator. This system free of a polar substance, however, has a low polymerization rate and moreover, a polymer obtained using it does not have sufficiently narrow molecular weight distribution.
It is already known that use of durene, which is a xcfx80 electron donor, in anionic polymerization changes the polymerization rate of styrene. According to J. of Polymer Science, Part A, 3, 1037-1044 (1965), when the molar ratio of durene to an active terminal Li is 1:5, the polymerization rate of styrene becomes about twice the rate of a durene-free case, suggesting the formation of a xcfx80 complex by durene. It however includes neither an attempt to prepare a dilithium polymerization initiator by using durene nor an attempt to polymerize a conjugated diene by positively using a dilithium polymerization initiator prepared using durene in anionic polymerization.
An object of the present invention is to provide a dilithium polymerization initiator solution which is useful for the preparation of a block copolymer having narrow molecular weight distribution and excellent symmetric property or the preparation of a telechelic polymer having, equally at both terminals thereof, a functional group, and which is free of a nitrogen- or oxygen-containing polar substance. Another object of the invention is to prepare a (co)polymer having narrow molecular weight distribution by using the above-described polymerization initiator solution as it is. A further object of the invention is to obtain, when a conjugated diene is employed as a monomer, a polymer having a low content of a 1,2-bound structure in the conjugated diene polymer portion. A still further object of the invention is to obtain a novel telechelic polymer by reacting the (co)polymer with a functional-group-imparting compound.
The present inventors have carried out an extensive investigation with a view toward overcoming the above-described drawbacks of a dilithium polymerization initiator solution and production of a (co)polymer using the solution. As a result, they succeeded in the completion of the following invention.
In one aspect of the invention, there is thus provided a polymerization initiator solution which comprises a dilithium polymerization initiator having an average number of functional groups of from 1.8 to 2.0/molecule and a solvent selected from aliphatic hydrocarbon compounds and alicyclic compounds, and which is free of a polar substance and a conjugated diene.
In another aspect of the invention, there is also provided a production process of the above-described polymerization initiator solution, which comprises reacting, in a solvent selected from aliphatic hydrocarbon compounds and alicyclic compounds, a mono-organo lithium compound and a compound having two vinyl groups directly bonded to a benzene ring in the presence of at least one compound selected from benzene compounds substituted with at least three alkyl groups and 1,1-diphenyl-substituted ethylene compounds.
In a further aspect of the invention, there is also provided a production process of the above-described polymerization initiator solution, wherein the benzene compound substituted with at least three alkyl groups is 1,2,4,5-tetramethylbenzene (durene) or hexamethylbenzene and the 1,1-diphenyl-substituted ethylene compound is tetraphenylethylene.
In a still further aspect of the invention, there is also provided a production process of the above-described polymerization initiator solution, wherein the compound having two vinyl groups directly bonded to a benzene ring is 1,3-diisopropenylbenzene or divinylbenzene.
In a still further aspect of the invention, there is also provided a production process of a co(polymer), which comprises polymerizing a conjugated diene or a conjugated diene and an aromatic vinyl compound in a nonpolar hydrocarbon solvent by using the above-described polymerization initiator solution.
In a still further aspect of the invention, there is also provided a copolymer obtained by the above-described process.
In a still further aspect of the invention, there is also provided a telechelic polymer obtained by reacting a functional-group-imparting compound with the active terminal of the (co)polymer polymerized by the above-described process.
The term xe2x80x9caverage number of functional groupsxe2x80x9d as used herein means a hydroxyl value measured in accordance with the following method. By using a polymerization initiator solution to be measured, 1,3-butadiene is polymerized and the resulting living polymer is reacted with 2 equivalents of dry ethylene oxide. After the reaction mixture is allowed to stand for 24 hours at room temperature, a 1N aqueous HCl solution is added to neutralize the reaction mixture. The neutralized mixture is washed with water and then separated to obtain an organic layer. The solvent is removed by thin-film evaporation, whereby a liquid oligomer is obtained. The hydroxyl value of this oligomer measured in accordance with JIS K 0070-1992 is designated as the average number of functional groups. The average number of functional groups of the dilithium polymerization initiator solution of the invention is 1.8 to 2.0/molecule, preferably 1.85 to 2.0/molecule, more preferably 1.9 to 2.0/molecule. The average number of functional groups within the above-described range indicates uniformity of the dilithium polymerization initiator. As the number approaches 2.0/molecule, the block copolymer obtained by using the initiator shows better symmetrical property and better mechanical properties. Similarly, use of this polymerization initiator makes it possible to provide a telechelic polymer having, equally at both terminals thereof, functional groups.
The dilithium polymerization initiator solution of the invention is obtained, for example, by reacting an mono-organo lithium compound and a compound having two vinyl groups directly bonded to a benzene ring at a molar ratio of 2:1 in an aliphatic hydrocarbon compound or alicylic compound in the presence of a benzene compound substituted with at least three alkyl groups or a 1,1-diphenyl-substituted ethylene compound.
As the solvent for the polymerization initiator solution, C4-8 aliphatic hydrocarbons, alicyclic hydrocarbons and mixtures thereof are usable, with the cyloalkanes, particularly cyclohexane and cyclopentane, being preferred.
Upon production of the dilithium polymerization initiator, a polar substance, for example, a tertiary amine such as triethylamine or trimethylamine or an ether compound such as THF should not be incorporated. Such a substance causes various problems during polymerization as described above. The present invention is characterized by that a uniform dilithium polymerization initiator free of problems due to a polar substance can be provided.
Examples of the mono-organo lithium compound include n-butyl lithium, sec-butyl lithium, tert-butyl lithium, 2-methyl-propyl lithium and i-propyl lithium. Of these, sec-butyl lithium and tert-butyl lithium are preferred, with tert-butyl lithium being most preferred.
The term xe2x80x9ccompound having two vinyl groups directly bonded to a benzene ringxe2x80x9d means a compound having two vinyl groups on the same benzene ring or different benzene rings. Examples of it include compounds represented by the following formula: 
wherein, R represents a hydrogen atom or a group selected from methyl, ethyl, phenyl and alkyl-substituted phenyl groups, and (Ar) represents a divalent aliphatic alicyclic or aromatic group, with the proviso that when (Ar) is an aliphatic or alicyclic group and is bonded to a carbon-to-carbon double-bond, R represents a phenyl or alkyl-substituted phenyl group.
Specific examples of (Ar) include p-phenylene, m-phenylene, p,pxe2x80x2-phenylene ether, p,p-diphenylene, p,pxe2x80x2-diphenylpropane and 1,8-octylene group.
Specific examples of such a compound having two vinyl groups directly bonded to a benzene ring include 1,3-diisopropenylbenzene, divinylbenzene, 1,3-di (1-phenylethenyl)benzene, 1,3-di [1-(methylphenyl)ethenyl)]benzene and 2,11-diphenyl-dodeca-1,11-diene. Of these, 1,3-diisopropenylbenzene and divinylbenzene are especially preferred.
Examples of the benzene compound substituted by at least three alkyl groups include 1,2,4,5-tetramethylbenzene (durene), hexamethylbenzene and 1,3,5-trimethylbenzene (mesitylene), of which the 1,2,4,5-tetramethylbenzene (durene) is particularly preferred. As the 1,1-diphenyl-substituted ethylene compound, tetraphenylethylene is preferred. These compounds are presumed to take part in association with an organolithium compound or a lithium ion exhibiting polymerization activity and have action to partially dissociate the association.
These benzene compound substituted with at least three alkyl groups and 1,1-diphenyl-substituted ethylene compounds are each added at a molar ratio of 1/10 to 1000/1 relative to the mono-organo lithium compound. The optimum range differs with the compound employed. For example, a molar ratio when durene is employed is 1/10 to 1000/1, which is however limited by the solubility in a solvent to be employed. The reaction temperature may be xe2x88x9220 to 60xc2x0 C., preferably 20 to 50xc2x0 C.
Raw materials and solvent used for reaction are usually employed after thorough purification. In addition, monomers employed for polymerization are also used after purification as needed. Impurities brought by a reaction container are also taken into account. Anyway, it is important to regulate a molar ratio of the mono-organo lithium compound to the compound having two vinyl groups directly bonded to a benzene ring to approximate to 2/1. The concentration of the dilithium compound obtained by the reaction can be determined as needed, but 0.0001 to 0.1 mole/liter is preferred. At a concentration less than this lower limit, fluctuations due to impurities become large. At a concentration greater than this upper limit, reaction does not proceed uniformly. Concentrations outside the above range are therefore not preferred. The dilithium polymerization initiator prepared within this concentration range can be employed after diluted with a nonpolar hydrocarbon solvent as needed.
The dilithium polymerization initiator solution thus obtained can be used as is for polymerization of a conjugated diene monomer, aromatic vinyl compound monomer or another anionic polymerizable monomer. By this polymerization, a homopolymer, block copolymer or random copolymer can be obtained. Typical examples of these monomers include, as a conjugated diene, butadiene, isoprene, 2-ethyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 1,3-pentadiene and 1,3-cyclohexadiene; as an aromatic vinyl compound, styrene, xcex1-methylstyrene, p-methylstyrene, m-methylstyrene, o-methylstyrene, p-tert-butylstyrene, divinylbenzene, 1,1-diphenylethylene and vinyl naphthalene; and, as another monomer, (meth)acrylates, vinylpyridine and xcex5-caprolactone.
Anionic polymerization by using the dilithium polymerization initiator solution of the invention is conducted under ordinary polymerization conditions, more specifically, at 0 to 120xc2x0 C. Polymerization of a conjugated diene at a higher temperature tends to readily generate a gel owing to a branched structure, though it produces a polymer poor in a 1,2-vinyl structure. Lower temperatures, on the other hand, retard the polymerization rate. The amount of each of the dilithium polymerization initiator and monomer is determined by the molecular weight of the polymer to be produced, viscosity of the polymer solution, collecting method of the polymer or the like.
Upon polymerization of a conjugated diene, a polar substance is unnecessary. If polymerization of an aromatic vinyl compound follows polymerization of a conjugated diene, addition of a small amount of a polar substance is required. Examples of the polar substance include ether compounds such as THF, diethoxypropanes, glyme and diglyme, and tertiary amines such as triethylamine and tetramethylethylene diamine. Of these, THF is preferred.
Polymerization of a block copolymer using a dilithium polymerization initiator is conducted by one of the following two methods: polymerization by successively adding monomers and polymerization utilizing a reactivity ratio of monomers. In the latter method, for example, a conjugated diene and an aromatic vinyl compound are charged simultaneously in a reactor containing a dilithium polymerization initiator. First, polymerization of the conjugated diene occurs. Then, a copolymerized portion of the conjugated diene and aromatic vinyl compound is formed, followed by the formation of a homopolymer of the aromatic vinyl compound. Accordingly, the block copolymer obtained using the dilithium polymerization initiator has a conjugated diene polymer block at the center and a homopolymer block of the aromatic vinyl compound at the terminal. The block copolymer obtained by using the dilithium polymerization initiator of the invention in accordance with this method differs from a block copolymer obtained using a conventional dilithium polymerization initiator prepared in the presence of a polar substance in that a portion of the random copolymer becomes smaller and a portion of the homopolymer of an aromatic vinyl compound (a portion which can be detected and determined as the homopolymer of an aromatic vinyl compound by osmic acid decomposition) becomes larger. Accordingly, the invention is characterized by that a polymer having a high tensile strength can be obtained. Use of the dilithium polymerization initiator solution of the invention makes it possible to prepare a block copolymer of a conjugated diene, for example, a block copolymer of butadiene and 1,3-cyclohexadiene.
A (co)polymer obtained by using the dilithium polymerization initiator solution of the invention features its narrow molecular weight distribution. The (co)polymer usually has a molecular weight distribution of 1.0 to 1.3. In the case of a conjugated diene polymer, a conjugated diene polymer ranging from that having a 1,2-vinyl-bound content suppressed to the minimum to that having an increased 1,2-vinyl-bound content can be obtained because the polymerization initiator solution is free of a polar substance. In addition, in the case of a block copolymer, a block polymer thus obtained has excellent symmetrical property. The copolymer therefore has a high tensile strength.
The co(polymer) obtained by the polymerization using the dilithium polymerization initiator solution of the invention has, as both terminals thereof, a lithium anion terminal having polymerization activity. A telechelic polymer can be produced by chemically modifying these active both terminals. Chemical modification of the terminal of a polymer by reacting a lithium anion terminal having polymerization activity with a functional-group imparting compound is a known method. In the invention, production of a telechelic polymer may follow a known method. Specific examples of the functional-group imparting compound to be used for the production of a telechelic polymer include ethylene oxide, propylene oxide, carbon dioxide, methacrylic acid derivatives, divinylbenzene, iodine, ethylenediamine, diglycidylamino-containing compounds such as tetraglycidyl-1,3-bisaminomethylcyclohexane and tetraglycidylmethaxylenediamine, N-methyl-2-pytrrolidone and 1,3-dimethyl-2-imidazolidione. Reaction conditions may be the same as known conditions. For example, a reaction with ethylene oxide yields a telechelic polymer having a hydroxyl group at both terminals. A reaction with carbon dioxide and hydrolysis yield a polymer having a carboxylic acid group at both terminals. By this method, a polymer having less content of the 1,2-vinyl structure and having a functional group at both terminals can be synthesized.
According to the invention, a uniform dilithium polymerization initiator solution free of a polar substance such as amine can be obtained and, by the use of this solution as it is, a polymer having narrow molecular weight distribution can be easily prepared without the need of forming an oligomer as required in the conventional process.
The present invention will be illustrated in greater detail with reference to the following Examples, but the invention should not be construed as being limited thereto.
The solvent, 1,3-diisopropenylbenzene, durene and monomer were purified by dehydration using a metal sodium, metal potassium, tert-butyl lithium or oligomer lithium and distillation under high vacuum conditions. Tetraphenylethylene was purified by conducting recrystallization and sublimation in repetition. In particular, the consumption of the monomer by polymerization reaction was suppressed by conducting distillation speedily. The concentration of tert-butyl lithium in the purchased cylohexane solution (product of FLUKA) of tert-butyl lithium was determined from the UV absorption spectrum of the active terminal of the polymerized styrene.